1. Field of the Invention
This invention is directed to microaccumulators for fluid under pressure and, in one aspect, to apparatuses using such devices in quantitative determinations of the strengths and mechanical properties of rock, including low permeability rock such as shale.
2. Description of Related Art
Many prior art pressure generators have a relatively long piston which moves in a small diameter bore. Hence, they have a small bore-diameter-to-piston-stroke length ratio; e.g. 1 to 4; 1 to 7; 1 to 8. Packing around such pistons can be compressed causing leaks. Many larger prior art accumulators require the use of larger linear variable differential transformers (LVDT's; e.g. products of Schaevitz Co. such as HR2000) which are not sensitive to very small volume changes.
In a conventional triaxial test apparatus for rock testing, the sample is deformed by gradually increasing the axial load until the ultimate strength of the specimen is reached at which point it fails; i.e., it will not sustain any further increased axial loading. Water or fluid can drain out of the sample during the test. The pore pressure within the sample is measured by an external pressure gauge or a more accurate pressure transducer via a pore pressure line through the end cap to the rock face. Pore pressure can be applied and controlled externally during a test by pumping water into the sample through the pore pressure line. The loading stage of the conventional test can be relatively fast and typically at high strain rates in the range of 10.sup.-2 to 10.sup.-4 sec.sup.-1 and sometimes as slow as 10.sup.-5 to 10.sup.-6 sec.sup.-1 and still achieve pore-pressure equilibrium during the test. Additionally, highly accurate control of the strain rates is not critical and usually not achieved for the typical high strain rates used for higher permeability rocks. An LVDT (linear variable differential transformer) on the load piston has been used to control the strain rate, however more recent applications have used LVDT's on the rock or end caps to obtain more precise control.
The prior art teaches pore pressure measurement for shales in tests with a triaxial end cap design that incorporates an accurate miniaturized pressure transducer into the end cap near the rock face which prevents drainage of the pore water from the rock during the test and which obviates external contact with water from a pore pressure line and thus preserves the original water content of the test specimen throughout the test.
There has long been a need for a microaccumulator for fluid under pressure to receive very small amounts of fluid. There has long been a need for such a device which can be used with triaxial test equipment to sense small amounts of fluid expelled from a test sample. There has long been a need for such a microaccumulator that can be used in triaxial tests of low permeability rocks, e.g. shales.